Such a fuel-metering system is known from U.S. Pat. No. 4,242,991. In the case of this fuel-metering system, during the overrun operation of the engine, that is, when the speed with closed throttle flap exceeds a certain value, the fuel feed is entirely switched off or at least strongly throttled. After the end of the overrun operation, in particular after the end of a lengthy overrun operation, the engine along with the intake pipe may have cooled to such an extent that part of the fuel condenses on the inside surface of the intake pipe and cylinder. This part of the fuel is lost from the ignition mixture, which thus becomes too lean. As the consequence of this, the engine does not run smoothly, the speed dips excessively or the engine cuts out. In addition, hydrocarbon emission peaks occur in the exhaust gas.
In order to achieve the rapid build-up of a fuel film on the wall after overrun cutoff without impairing the quantitative balance of the metered fuel on the one hand and of the mixture fed to the cylinder for combustion on the other hand, with overrun cutoff the steady fuel quantity metered from the metering system, corresponding to the operating point of the engine, is enriched by a predetermined extra quantity of fuel. The extra quantity may in this case be constant over a certain number of ignition pulses and metering pulses coupled therewith, or may be varied with each metering pulse.
Even with variation of the extra quantity with each metering pulse, the quantity of fuel metered as a whole can always only be an inadequate compromise due to the complex interrelationships of the necessary additional wall-film quantity and the associated operating collective prehistory of the engine. The extra quantity may be too great or too small. This has the corresponding effects. An excessively rich mixture leads to carbon monoxide exhaust gas peaks, a still too lean mixture leads to hydrocarbon emission peaks and to cutting-out of the engine.